Abstract

Eddycurrents induced by applied magnetic-field pulses have been a common issue in ultra-low-field magnetic resonance imaging. In particular, a relatively large prepolarizing field—applied before each signal acquisition sequence to increase the signal—induces currents in the walls of the surrounding conductive shielded room. The magnetic-field transient generated by the eddycurrents may cause severe image distortions and signal loss, especially with the large prepolarizing coils designed for in vivo imaging. We derive a theory of eddycurrents in thin conducting structures and enclosures to provide intuitive understanding and efficient computations. We present detailed measurements of the eddy-current patterns and their time evolution in a previous-generation shielded room. The analysis led to the design and construction of a new shielded room with symmetrically placed 1.6-mm-thick aluminum sheets that were weakly coupled electrically. The currents flowing around the entire room were heavily damped, resulting in a decay time constant of about 6 ms for both the measured and computed field transients. The measured eddy-current vector maps were in excellent agreement with predictions based on the theory, suggesting that both the experimental methods and the theory were successful and could be applied to a wide variety of thin conducting structures.

We are grateful to Steven Conolly for providing the water-cooled polarizing coil used in the system. We thank Matthew Nichols and Kevin Lee for technical assistance. This research was supported by the National Institutes of Health Award No. 5R21CA1333338 and by the Donaldson Trust. This work also received funding from the Academy of Finland, from the Finnish Cultural Foundation and from the European Community's Seventh Framework Programme (FP7/2007–2013) under Grant Agreement No. 200859.